Inconsistent ignition of air/fuel mixtures in internal combustion engines can result in variations in the ignition delay and burn duration of the fuel mixture in a cylinder. Such inconsistency can lead to significant variation in peak pressures and temperatures within the cylinder and can, in knock limited engines, limit the spark timing and/or compression ratio, thereby complicating accurate setting of the spark timing close to the knock limit. Variations in burn speed and duration of the burn can also translate directly into variations in efficiency. Typical coefficients of variation in indicated mean effective pressure (IMEP), which represents the average pressure over a cycle in the combustion chamber of a cylinder, can be approximately 1.5%. In other words, assuming a normal distribution, the least effective three cycles out of a thousand cycles with the same fuel input generate 9% (6×1.5%) less work than the most effective three cycles out of the thousand. Moreover, if the ignition must be retarded to avoid knock in the quickest burning cycles, ignition timing can be compromised on the remaining slower burning cycles which can compromise torque and efficiency.
Slow flame speed can also limit how late spark ignition can be initiated in the combustion cycle while still achieving a complete burn. The effectiveness of currently available high swirl engines is often limited by the high heat transfer rate between the gases in the cylinder and the walls of the cylinder that results from the high velocity gas motion created in the cylinder. The effectiveness of traditional engines with high velocity gas motion can also be impacted by the pressure drop in the intake passage necessary to induce a high gas velocity. This pressure drop can limit the maximum flow of air through a cylinder of the engine and hence the maximum power achievable. Also, with additional combustion stability, a leaner mixture can be ignited for a given coefficient of variation of IMEP.